Thermal neutron three-axis spectrometer with polarization analysis IN20
Reactor hall, thermal beam H13
beam tube Ø 170 mm
|crystal||dhkl (Å )||ki (Å-1)||flux (107n.cm-2s-1)|
horizontal and vertical focusing
4 x 10 cm2 (vertical)
20 x 20 mm2
IN20 is installed at the H13 thermal beam tube (Ø 170 mm) in the reactor hall. Both the primary and the secondary spectrometer employ a monochromatic horizontal focusing geometry. A heavy input slit of an adjustable size, placed in the casemate, serves as a virtual source, providing a large solid angle for the monochromatic beam, while reducing, together with a sapphire filter window, the fast neutron background. The neutron energy is selected either by a doubly focusing polarizing Heusler 111 monochromator (230 x 150 mm2 w x h) or by an unpolarised Si 111 monochromator (elastically bent crystals, 195 x 200 mm2 w x h) free of higher-order contamination in the incident beam at wave-numbers ki > 3 Å-1. The analysis of the energy and polarisation state of the scattered neutrons is effectuated by a similar horizontally focusing Heusler crystal analyzer. Further PG 002 and Si 111 analyzers are available for occasional unpolarised work.
The energy transfer range accessible in the present configuration of IN20 extends to 100 meV with maximum incident neutron energies reaching 150 meV. The typical energy widths (FWHM) measured with a reference vanadium sample at the graphite filter wave-numbers ki = 2.66 Å-1and 4.1 Å-1 are 0.82(3) meV and 3.05 (15) meV, respectively.
Polarisation analysis can be performed in combination with the standard ILL 2.5 T and 6 T vertical-field and 4 T horizontal-field cryomagnets as well as with a set of Helmholtz coils generating a guide field (about 15 Gauss), which automatically follows the direction changes of the scattering vector. This coil set can receive the standard ILL cryostats and furnaces. The overall beam polarisation is about 90% (flipping ratio 20) in most of the spectrometer confi- gurations. Thanks to the recent upgrade a single crystal volume of 0.1 – 1 cm3 is sufficient for investigations of most systems of current interest.
- Zero field neutron polarimetry - CRYOPAD - the sample region is shielded from external magnetic fields by superconducting foils so that the sample space is field-free. Cryopad is therefore very useful for polarisation analysis of superconducting samples.
It further allows to determine the complete polarisation transfer matrix.
- FlatCone multianalyser - is available with kf = 3 Å-1 for mapping the inelastic response in unpolarised mode at energy transfers up to 35 meV.